Apparatus for monitoring one or more surgical parameters of the eye

ABSTRACT

An apparatus for monitoring one or more surgical parameters of the eye over multiple sessions which are temporally spaced apart and between which the eye of the patient can have moved, said apparatus comprising: a camera for taking one or more images of the eye; a module for determining during a first session said at least one surgical parameter of the eye and its coordinates based on the image taken by said camera in a first coordinate system; a module for determining during a second session temporally spaced apart from said first session said at least one surgical parameter of the eye and its coordinates based on the image taken by said camera in a second coordinate system; a module for determining the eye motion in six degrees of freedom between said first and said second session and for determining a coordinate transformation based thereon; a module for transforming based on said determined eye motion said at least one surgical parameter of the eye and its coordinates from said first coordinate system into said second coordinate system; a module for quantifying and/or visualizing the change of said at least one surgical parameter of the eye and its coordinates between said first and said second session based on said surgical eye parameter and its coordinates measured during said second session and said transformed surgical eye parameter and its coordinates measured during said first session, wherein said surgical eye parameters are one or more of the following: implant-related parameters of the eye which are based on an implant which has been surgically placed in the eye of a patient; or the location and/or contour of corneal or limbal or scleral incisions.

FIELD OF THE INVENTION

This Invention relates to an apparatus for monitoring one or surgicalparameters of the eye.

BACKGROUND OF THE INVENTION

The invention refers to the field of ophthalmology, specificallyrefractive eye diagnostic and eye surgery. For most refractive eyetreatments

(1) pre-surgery diagnostic information of the patient's eye isdetermined to choose the adequate procedure (e.g. implant vs. laser) anddefine the individual treatment steps (e.g. where to cut or how to alignthe implant),

(2) the individual surgery treatment is performed inserting refractioncorrecting implants (e.g. IOL's, corneal inlays) or executing surgeryactions (e.g. cut incisions, apply laser shot patterns) and

(3) post-surgery diagnostic information of the patient's eye includingimplant and/or surgery action is determined.

(1) and (3) are typically performed outside the operation room usingdiagnostic devices like keratometer, topographer, wavefront analyzer,scheimflug devices, interferometer or slit lamps. (2) is typicallyperformed in the operation room using a general purpose surgicalmicroscope and adequate tools to support the surgeons manual work (e.g.knifes, phaco machine) or using dedicated devices for partial or fullautomation of surgical steps (e.g. refractive excimer laser treatment,cataract laser treatment).

Currently there is a wide range of diagnostic devices that measureproperties of the eye. A topograph or keratometer determines the shapeand curvature of the patient's cornea (e.g. Zeiss Atlas), a wavefrontdevice determines the full refraction of the patient's eye optics (e.g.AMO Wavefront Sciences COAS), an interferometer measures the axiallength of the patient's eye ball (e.g. Haag-Streit LenStar LS900), ascheimflug device measures the front-side and back-side of the cornealrefraction as well as the thickness (e.g. Oculus Pentacam) and a slitlamp provides an image of the patient's front of the eye for manualexamination by the doctor.

All different diagnostic approaches and associated devices evolved toaccurate tools with a high repeatability for single eye measurements andtherefore are applied pre-surgery as well as post-surgery forexamination to verify clinical outcome.

There are further approaches appearing on the ophthalmology landscapefor intra-surgery measurement of the eye. An intra-surgery keratometryhand tool (e.g. astigmatic ruler by STORZ) can be used to roughlymeasure the corneal shape and its changes during the surgery, anintra-surgery wavefront device—in principle—allows the determination ofthe required power and astigmatism of an artificial lens after theremoval of the natural lens (e.g. Wavetec ORange). All intra-surgeryrefraction measurement tools suffer from the moment of taking themeasurement: The moment of eye surgery. Intra-surgery the eye propertiesare changed compared to the natural no-surgery condition. The intraocular pressure might be higher, the cornea might be deformed due tomechanical impacts, the refraction of eye fluids changed due to partialexchange of fluids, etc. But independent from this general drawback, therepeatability of those devices in one moment on one specific eye isreasonable.

All named devices and tools in this section above have in common theavailability of a more or less consistent intra-device coordinate system(“device-consistent” which means that the tool or device provides from apatient X measured at one moment T multiple times a consistent output)but they all lack a full process covering consistent coordinate system(“process-consistent”). With a process-consistent coordinate systemevery process step (measurement or treatment) where the patient's eye isvisually acquired, can be matched and transformed to an initiallydefined reference coordinate system.

Due to the lack of a process-consistent coordinate system, systematicerrors that occur between different steps are directly impacting theoverall treatment error. Some examples:

a) Sit-to-Sit-Error: Current practice is making all diagnosticmeasurements with the patients head is in an upright position. Theassumption of 99% of surgeons is that the gravitation keeps the eye inthe exact orientation for every measurement. This way a combination ofmeasurement results from different devices can easily be performed.Unfortunately this assumption is wrong. The eye can rotate up to 7° fromone sitting position to another.

b) Marker-Error: Current practice is the use of ink markers or inkmarker tools for marking axes or positions on the cornea or the limbusborder. The accuracy for using ink markers is limited due to the size ofthe marker (e.g. can be a 5° thick mark), the unknown coordinate systemwhile the surgeon is doing the marking (see a)) as well as the accuracyof reading a marker. The errors can easily sum up to 6° or more.

c) Surgeons-Error: Till now e.g. the cataract surgeon is doing mostsurgery steps that require special accuracy fully manual: They positionincisions or align implants based on the marks they did previously.Besides the Maker Error the mechanical precision of the surgeon fingersneeds to be taken into account.

d) Implant-Error: Depending on the type of implant differentpost-surgery movements of the implant are likely to occur. For exampleearly toric IOL designs tend to move post-operatively up-to 10° based onslit lamp assessment.

Deriving guidelines, nomograms or new implant designs and tool designsfrom the overall clinical outcome a separation of different systematicerror influences like a)-d) could not be determined or distinguished.

With the high optical complexity of latest generation implants or latestgeneration laser systems this demand for more diagnostic and surgeryaccuracy is already present, but with existing tools only overall errorscan be determined but no error propagation addressing every singlediagnostic step or surgery step.

SUMMARY OF THE INVENTION

In view of the foregoing situation, according to one embodiment there isprovided a process-consistent coordinate system every process step(measurement or treatment) where the patient's eye is visually acquired,can be matched and transformed to an initially defined referencecoordinate system. This overcomes the disadvantages of the lack of acoherent process coordinate system over multiple sessions which maycomprise pre-surgery, surgery and post surgery.

According to one embodiment an apparatus is provided for monitoring oneor more surgical parameters of the eye of a patient over multiplesessions which are temporally spaced apart and between which the eye ofthe patient can have moved, said apparatus comprising:

a camera for taking one or more images of the eye;

a module for determining during a first session said at least onesurgical parameter of the eye and its coordinates based on the imagetaken by said camera in a first coordinate system;

a module for determining during a second session temporally spaced apartfrom said first session said at least one surgical parameter of the eyeand its coordinates based on the image taken by said camera in a secondcoordinate system;

a module for determining the eye motion in six degrees of freedombetween said first and said second session and for determining acoordinate transformation based thereon;

a module for transforming based on said determined eye motion said atleast one surgical parameter of the eye and its coordinates from saidfirst coordinate system into said second coordinate system;

a module for quantifying and/or visualizing the change of said at leastone surgical parameter of the eye and its coordinates between said firstand said second session based on said surgical eye parameter and itscoordinates measured during said second session and said transformedsurgical eye parameter and its coordinates measured during said firstsession, wherein said surgical eye parameters are one or more of thefollowing:

implant-related parameters of the eye which are based on an implantwhich has been surgically placed in the eye of a patient; or

the location and/or contour of corneal or limbal or scleral incisions.

Such an arrangement allows to monitor surgical parameters even after thesurgery has been performed to check whether there has been any temporalchange of the surgical parameters like implant-related eye parameters orthe location or contour of incisions. This is an important diagnosticinformation for monitoring the success or failure of surgery during thepost-surgical phase.

According to one embodiment said implant related eye parameter comprisesone or more of the following:

the orientation and/or position of the implant in the eye;

the location and/or the contour of the rhexis;

the overlap of the rhexis with the contour of the implant.

These are preferable examples of implant-related parameters for which itis interesting to monitor them to watch the surgical result over time.

According to one embodiment said module for quantifying and/ordisplaying the change of said at least one surgical eye parametercomprises:

A module for displaying said least one surgical eye parameter measuredduring said second session and said transformed surgical eye parametermeasured during said first session in the image of the eye taken duringsaid second session; and/or

a module for calculating the difference between said surgical eyeparameter measured during said second session and said transformedsurgical eye parameter measured during said first session and forvisualizing said difference in said image of the eye taken during saidsecond session.

This enables the comparison of the development of a surgical parameterover time, e.g. by comparing a post-surgical change with the situationduring surgery, or by comparing two different post-surgical instances intime while the eye movement between the two measurements is compensated.The surgical parameter (like the location of the implant) as determinedat the two instances of time may be directly visualized by displaying itin the image with the eye motion being compensated, or there may becalculated a difference (like a difference in x-, y- or rotationparameters) and just the difference being displayed in the image.

According to one embodiment said first session is a pre-surgery sessionand said second session is an intra surgery session or a post surgerysession, or

said first session is an intra-surgery session and said second sessionis a post surgery session, or

said first session is a post-surgery session and said second session isanother post surgery session performed at a later time. These aresuitable examples of sessions at different instances of time for whichthe surgical parameters may be compared while compensating for the eyemotion between the sessions.

According to one embodiment the apparatus further comprises:

A module for measuring and recording said at least one surgical eyeparameter during multiple sessions over time in order to record thechange of said at least one surgical eye parameter over time.

This enables the recording and monitoring of the development of surgicaleye parameters and thereby of the surgical result over an arbitrarilylong time period in a consistent coordinate system by compensating theeye motion. In this way e.g. studies regarding the long term success orfailure of surgical techniques may be carried out which so far are notpossible.

According to one embodiment the apparatus further comprises: anillumination unit for illuminating the eye by a ring-shaped lightpattern to generate corneal reflection, said illumination unit beingpreferably located such that the center of the ring is coaxial with theoptical axis of the camera;

a module for determining during said first and/or said second sessionthe location of the corneal reflections in the image of the eye;

a module for determining during said first and/or said second sessionbased on said determined location of the corneal reflections, at leastone further parameter of the eye and its coordinates in said firstand/or second coordinate system based on a geometrical modelrepresenting the eye as a spherical eyeball having a spherically shapedcornea mounted thereon; a module for visualizing said at least onefurther parameter of the eye together with said at least one implantrelated eye parameter in the same image after having transformed itscoordinates based on the eye motion between said first and said secondsession so that the motion of the eye is compensated.

In this way further eye parameters which are not surgical eye parametersmay be determined and monitored in addition to the surgical eyeparameters. They may also be visualized additionally together with thesurgical eye parameters while compensating for the eye motion betweenthe sessions.

According to one embodiment said at least one further parameter isdetermined based on an eye model which represents the shape and locationof the eye by a spherical eyeball and a cornea mounted thereon andhaving a spherical shape or the shape of an ellipsoid to thereby enablethe calculation of said at least one further parameter using themeasured location of said corneal reflections and said eye model.

This is a suitable way of determining eye parameters which are notdirectly measurable from the images taken by a camera.

According to one embodiment said at least one further eye parametercomprises one or more of the following:

a) the k-readings which define the shape of the cornea in terms ofrotation ellipsoid parameters;

b) the line of sight as the line connecting the pupil center and afixation point of known location;

c) the corneal chamber depth;

d) the visual axis of the eye;

e) the determination whether the eye is the left eye or the right eye.These are suitable examples of further eye parameters.

According to one embodiment said at least one further eye parametercomprises the k-readings which are measured by determining a best fitellipse to the corneal reflections and determining the major axis, theminor axis and the orientation of the ellipse.

This enables the determination of astigmatism parameters including thelength of the steep and flat axis of the cornea as well as theorientation of the astigmatism. The diameter of the best fit corneasphere can be approximated by the mean of flat and steep axis.

According to one embodiment said apparatus further comprises a fixationtarget at known coordinates, preferably on the optical axis of thecamera, and said at least one further eye parameter comprises the visualaxis which is determined as the vector connecting the cornea center andthe known fixation target, where the cornea center is determined basedon the location of the corneal reflections.

This enables the determination of the visual axis. According to oneembodiment said at least one further eye parameter comprises the anglekappa between the visual axis and the pupil axis, or

said further parameter is the intersection point between the visual axisand the cornea surface, where the cornea radius is determined based onthe location of said corneal reflections.

This allows the determination of further parameters which areinteresting for the surgeon.

According to one embodiment said at least one further eye parametercomprises the anterior chamber depth which is determined based ondetermining the radius of the limbus RI and assuming it to be a circleof latitude on the best fit cornea sphere with radius Rc which isdetermined based on the corneal light reflections such that the cornealchamber depth CD is derived by CD=Rc−sqrt(Rc^2−RI^2).

The anterior chamber depth is an interesting information for thesurgeon,

According to one embodiment said at least one further eye parametercomprises the line of sight which is determined as the vector connectingthe pupil center and said fixation point of known location, with thez-coordinate of the pupil center being determined based on a knowndistance between camera and the eye and the x- and y-coordinates of thepupil being determined based on measuring the pupil location in theimage, and/or

said at least one further eye parameter comprises the pupillary axisbeing the line going through the center of the pupil and beingorthogonal to the cornea surface.

Line of sight and papillary axis may be determined in this way.

According to one embodiment said at least one further eye parametercomprises the determination of whether the center of the limbus or thecenter of the cornea is closer to the optical axis of the camera whenthe patient fixates a known fixation point lying on the optical axis ofthe camera.

This enables the determination whether the eye is the left eye or theright eye. It may be used as a safeguard mechanism to prevent thesurgery or diagnosis being performed on the wrong eye.

According to one embodiment the apparatus further comprises:

A module for visualizing an arbitrary combination of said at least oneor more surgical eye parameters and said at least one or more furthereye parameters determined during said first session and a possiblydifferent arbitrary combination of said at least one or more surgicaleye parameters and said at least one or more further eye parametersdetermined during said second session in the same image such that theeye motion between said first and second session is compensated.

This allows the visualization of any surgical or other parameters in anycombination which are of interest while compensating for the eye motionbetween different sessions.

DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 15 illustrate embodiments of the invention.

DETAILED DESCRIPTION

According to one embodiment there is provided an apparatus which enablesa solution for monitoring eye properties related to eye surgery overtime, between any two of the following:

pre surgery

intra surgery

post surgery

In the following there will be referred to spatial and refractive eyeproperties as “eye parameters”.

For intra surgery measurements the solution according to one embodimentrequires a microscope camera that is connected to a PC.

For pre and post surgery measurements according to one embodiment thesolution described here uses a specific apparatus hereinafter called a‘Reference Device’ (RD) which consists of a PC connected to a digitalcamera and an illumination system on a cross table which allowscapturing a high resolution color image of a patients eye in a definedposition. The apparatus according to one embodiment and its use inconnection with an eye is schematically illustrated in FIG. 1.

The illumination system of the RD generates a ring-shaped illuminationpattern and may e.g. consist of a concentric ring of LEDs around theoptical axis of the camera and a fixation LED which is injected on theoptical axis of the camera. Preferably the ring of LEDs is coaxial withthe optical axis of the camera and the optical axis of the camera isorthogonal to the area of the ring.

The acquired images are processed on the PC and can be used toautomatically or manually measure either absolute eye parameters as theyare at the time of image acquisition or changes of eye parametersrelative to a reference image of a previous measurement session.

According to one embodiment the apparatus allows determining the spatialrelation of the measured parameters with respect to each other withinand between measurement sessions by actively measuring how the eye didmove in 6 degrees of freedom between 2 measurement sessions.

The eye motion in 6 degrees of freedom is according to one embodimentmeasured based on registration of scleral blood vessel features orlimbus, iris features and corneal reflections of a defined illuminationsystem between 2 sessions.

One initial (usually pre surgery, but post-surgery is also possible)reference measurement serves as a reference coordinate system for allsubsequent measurement sessions (pre or post surgery) of the same eye.

All parameters measured in subsequent sessions can be transformed intothe reference (or vice versa) coordinate system by applying a spatialsimilarity transformation that accounts for the eye motion between thecurrent measurement and the reference measurement. Once transformed tothe reference coordinate system the parameters from differentmeasurements can be compared and the influence of eye motion iseliminated.

This approach is used in one embodiment for analyzing parameters likethe position and the orientation of eye implants (e.g. IOLs) in the eye.This way it can be monitored how stable the implant is located andoriented in the eye over time without being limited in accuracy to theamount of eye motion between measurement sessions.

Typical eye parameters that may be measured with the RD in a pre surgeryreference measurement session are:

1) Pupil position, shape and size (photopic, scotopic, mesopic)

2) Limbus position shape and size

3) K-readings

4) Line of sight (LOS)

5) Approximation of corneal chamber depth

6) Intersection of LOS with cornea surface and angle kappa

7) OD/OS classification

These eye parameters may be measured in a pre-surgery session and thenlater in intra-surgery or a post-surgery session, and their change ordevelopment over time may then be determined and visualized.

The eye motion which then enables the transformation of the eyeparameters from one session to another according to one embodiment isdetermined by measuring the following:

8) Relative eye motion with respect to the reference measurement bymeasuring

a) Relative translations in X and Y

b) Relative translation in Z

c) Relative cyclotorsion (around Z axis)

d) Relative roll and tilt (around X and Y axis)

Other parameters which relate to ophthalmic surgery and the placement ofimplants may be measured as well.

In a (subsequent) intra or post surgery measurement session thefollowing eye parameters may be measured in addition to (or instead of)the aforementioned eye parameters:

9) Orientation and Position of implants in the eye

a) Location of the implant markings in the eye (toric marks ormultifocal rings)

b) Rotational orientation of implants

c) Roll and Tilt of implants

d) Implant contour

e) XY-Position of the implant center

f) Location of the implant haptics in the eye.

Moreover, another type of parameters which is also related to implantsmay be measured, namely

10) The Rhexis in the capsular bag, specifically

a) Contour

b) Diameter

c) XY Position in the eye

d) Overlap with lens

In an alternative instance the RD contains an additional Scheimpflug orinterferometer setup that allows to measure inside the cornea and lenstissue. In such a setup in addition to the parameters mentioned above,corneal incisions can be measured in terms of location in the eye, widthand depth as well as the distance of the implant to the cornea.

In a second alternative instance the RD also contains a placido ringillumination that allows to analyze the topography of the cornea. Insuch a setup the exact changes in corneal topography e.g. before andafter LASIK laser treatment can be assessed. By applying the spatialsimilarity transformation to the topography data it is possible toensure that the topography data is correctly aligned and changes in thetopography of the cornea are being calculated correctly.

In a third alternative instance the RD also contains a wavefrontanalyzer (Hartmann-Shack-Sensor) that allows to analyze the fullrefraction of the eye.

In yet another alternative instance a registration of the image from theRD is performed to other dedicated eye diagnosis devices allowing totransform the dedicated parameters measured by these devices to thereference coordinate system provided by the RD. In this instance changesin these additional spatial eye parameters can also be monitored overtime in the consistent reference coordinate system provided by the RD.

In the following embodiments of an apparatus according to the invention(a reference device) will be described and its operation and functionwill be explained.

The main functionality of the apparatus according to one embodiment isto:

measure multiple eye parameters or parameter sets in differentmeasurement sessions.

determine the eye motion between the measurement sessions.

apply a spatial similarity transformation to transform each eyeparameter or parameter set to the reference coordinate system defined bythe initial reference measurement.

quantify and display changes in eye parameters or eye parameter setsbetween measurement sessions pre-, intra- and post-surgery.

quantify and display differences between surgery plan and post surgeryoutcome.

The eye parameters in one embodiment are measured by combining imageprocessing with a generic eye model. For example, according to oneembodiment the model represents the eyeball as a sphere with the corneabeing also spherical (or in one embodiment having an ellipsoid shape)being mounted thereon. Using such an eye model allows to indirectlymeasure properties like the corneal chamber depth which is not directlyvisible in the image.

Now it will be explained how according to embodiments eye parameters aredetermined which may then be transformed from one session to the otherusing the detected eye motion

1) Pupil Position, Shape and Size (Photopic, Scotopic, Mesopic)

Pupil detection is a classic image processing task. A classic thresholdbased approach is used here. By varying the illumination intensity thepupil of the patient can be brought into a photopic, scotopic andmesopic condition (pupil size changes).

2) Limbus Position Shape and Size

Similar as for pupil detection a standard approach using limbal edgedetection and a circular fit is used here.

3) K-Readings:

The k-readings define the shape of the cornea in terms of rotationellipsoid parameters as minor axis (steep axis in ophthalmology) majoraxis (flat axis in ophthalmology) and axis orientation. Also here in oneembodiment a well known keratometry approach is being applied bydetecting the corneal reflections of the coaxial ring of LEDs of the RD.The best fit ellipse into these reflections gives the parameters of thek-readings.

4) Line of Sight (LOS)

The Line of sight connects the fixation point with the center of theeye's entrance pupil. The RD takes an image from a defined distance Zpto the eye. By design the imaging geometry of the camera is known aswell as the position of the fixation target with respect to theprojection center of the camera. The pupil can therefore be measured in3 dimensions with its coordinates Xp, Yp and Zp. The 3d vectorconnecting the entrance pupil and the fixation target gives the LOS.This is schematically illustrated in FIG. 2.

5) Approximation of Corneal Chamber Depth

The radius Rc of the best fit sphere resembling the cornea surface isthe mean of flat and steep axis as determined from the k-readings.Assuming the limbus with radius RI to be a circle of latitude on thebest fit cornea sphere with radius Rc, an approximation of the cornealchamber depth CD can be derived by CD=Rc−sqrt(Rc^2−RI^2). This isschematically illustrated in FIG. 3.

6) Intersection of LOS or Visual Axis with Cornea Surface

The intersection is a valid reference point for implanting cornealinlays and for centering laser treatments. It can be approximated byintersecting the best fit cornea sphere with the LOS.

The lateral coordinates of the center of this sphere Xc and Yc are wellapproximated by the center of the corneal reflections of the ring ofLEDs. The Z coordinate of the sphere center is modeled by Zc=Zp−CD+Rc.

Using simple vector algebra the intersection between the LOS and thesphere defined by its center [Xc,Yc,Zc] and its radius Rc can becalculated.

Implicitly this intersection is also a representation for the oftencited angle kappa or lambda. In the literature angle kappa is referredto as the angle between the Visual Axis (VA see definition in sectionbelow) and the Pupillary axis (PA) connecting the pupil center[Xp,Yp,Zp] to the cornea center [Xc,Yc,Zc]. The PA is therefore a normalto the cornea surface. This and its determination is illustrated in FIG.4. The determination of the PA may in one embodiment the carried out asfollows:

1. Detect Pupil center in image to get pupil XY

2. Detect corneal reflections

3. Calculate cornea center XYZ and Cornea radius from CRs

4. Detect Limbus size in image

5. Use limbus size and cornea radius to calculate anterior chamber depth

6. Use anterior chamber depth and cornea center XYZ to calculate pupil Z

7. PA is vector through pupil XYZ and cornea center XYZ

Since an objective measurement of the VA is not trivial often the LOS isused instead, its determination has already been described above. Theangle between PA and LOS is referred to as angle lambda in theliterature (see FIG. 5). In practical terms lambda=kappa (up to 0.2°).

However, according to one embodiment the actual visual axis may bedetermined. For that purpose it is in one embodiment assumed that thecornea center matches with the first nodal point. Then the visual axiscan be determined as the line connecting the fixation point and thecenter of the cornea. This is illustrated in FIG. 6. FIG. 7 thenillustrates the determination of the angle kappa. The determination inone embodiment may be carried out using the following steps:

1. Detect corneal reflections

2. Calculate cornea center XYZ and Cornea radius from CRs

3. Use model assumption cornea center=1st nodal point

4. Use given XYZ coordinates of fixation target

5. VA is vector through 1st nodal point XYZ and fixation target XYZ

7) OD/OS Classification:

Another parameter that can be derived from images acquired with the RDis whether the current image shows a left or a right eye. This parameteris rather interesting for usability purposes and gross error prevention.

It is well known in the literature that the Visual Axis (VA) (ray oflight that connects the fixation point with the fovea through the firstand second nodal point of the eye) has an inclination towards the nasalside compared to the Optical Axis of the Eye (OAE) (see image below).The angle between the OAE and the VA is referred to as angle ALPHA inthe literature and has a magnitude of about 5°.

The OAE is the best fit line through the centers of curvature of thebest fit spheres to the refractive surfaces of the eye. The refractivesurfaces are the front and back surface of the cornea and the front andback surface of the lens. By centering the patient's eye in the cameraimage and by asking the patient to fixate on the target, the patientroughly aligns the VA to the Optical Axis of the Camera (OAC). Hence theOAE has an angle of about 5° to the OAC. The center of the cornealreflection(s) resembles a very good approximation of the image positionof the cornea center which by definition of the OAE lies on, or veryclose to the OAE.

A new aspect utilized in this embodiment is that an axis connecting thelimbus center and the cornea center, which will be referred to as LimbusAxis (LA), also provides a very reliable and stable reference toquantify the inclination of the VA towards the nasal side. The OD/OSclassification based on the cornea center and the limbus center isreliable since:

The patient is fixating and aligns the VA to the OAC.

Both, the center of the cornea and the center of the limbus do lie onthe LA and very close to the OAE.

The limbus center is always closer to the camera than the cornea center.

The VA points to the nasal side.

This is illustrated in FIG. 8.

It follows that in the camera image the cornea center appears left ofthe limbus center for the left eye and right of the limbus center forthe right eye. This is illustrated in FIG. 9.

In the following there will be explained in somewhat more detail howaccording to one embodiment the eye motion is measured and thecoordinated transformation is determined.

According to one embodiment there is determined the relative eye motionwith respect to the reference measurement in 6 degrees of freedom. Thisis the basis for the link between measurements taken during differentmeasurement sessions that may be minutes, days, months or years apartand may be performed on different diagnostic devices. U.S. Pat. No.7,600,873 B2 teaches how to utilize eye features like sclera bloodvessels, pupil, limbus, iris features and/or corneal reflections forrecovering eye motion in 6 degrees of freedom.

The 6 recovered parameters (translations in X,Y,Z and rotations aroundX,Y and Z-axes) describe a transformation—a spatial similaritytransformation—that may be applied to any derived coordinates on the eyeor in the eye. In one embodiment the same feature based approach asdescribed in U.S. Pat. No. 7,600,873 B2 is used.

In the foregoing there have been described embodiments where eyeparameters which relate to the shape or location of the eye or itsoptical properties are determined by using an image of the eye and ofcorneal reflections of a ring shaped illumination source and eye modelwhich represents the eye itself by a geometrical model. In addition tothe corneal reflections which are directly determined, one or more ofsuch “further” eye parameters are determined using the eye model: thedetermined setting of the camera, the illumination source, and in someembodiments also comprises a known fixation point. These parameters aredetermined over multiple sessions to monitor and record the change ofthese parameters over time between different sessions by using acoordinate transformation which is based on the determination of the eyemovement in six dimensions. It should be noted that the described“further parameters of the eye” may be measured alone or in an arbitrarycombination in a measurement session.

Now embodiments will be described in which further surgical eyeparameters, e.g. eye parameters which relate to implants are determined,such as e.g. the orientation and/or position of implants. Theseparameters may be measured in addition to the “further eye parameters”described before, or they may be measured alone or alternatively to themduring one session. Like with the “further eye parameters” describedbefore these implant related parameters are measured during multiplesessions which are temporally spaced and between which the patient—andthe eye—typically has moved. Also for these “implant-related parameters”the movement of the eye between different sessions in six degrees offreedom is determined to obtain a transformation which enables thetransformation of the measured parameters into a consistent coordinatesystem which is consistent over the multiple sessions. This enables thento compare and monitor how these implant related parameters change overtime which is very important information for the doctor. For thatpurpose these parameters may be compared with their correspondingimplant-related parameters as determined in previous sessions, or withthe “further” non-implant related parameters. The parameters ofdifferent sessions (non-implant related ones, implant related ones orany combination of both of them) which are to be compared may bevisualized within the same image by using the coordinate transformationobtained by the eye movement determination which enables the doctor tojudge the development of these parameters over time in a consistentcoordinate system which compensates or eliminates the effect of themovement of the eye between different sessions.

Other surgical eye parameters which may be determined are e.g. thelocation and/or contour of corneal or limbal or scleral incisions. Theseparameters may have a relation with an implant (and may therefore insome embodiments be “implant-related parameters”), however, there arealso surgical techniques like e.g. the LRI (limbus relaxation incision)where incisions are made without an implant being placed. For suchsurgical techniques the relevant parameters like the location and/orcontour of corneal or limbal or scleral incisions may be determined overmultiple sessions.

In the following embodiments will be described where implant-related eyeparameters are determined. The implant related eye parameters may in oneembodiment belong to one of two categories, the first one being theposition and/or orientation of an implant in the eye, and the second onebeing related to the position and/or orientation of the rhexis.

Both may also be combined, for example the position of the rhexis andthe location or shape of a lens implant.

In the following some embodiments will be described in more detail.

First some embodiments measuring the orientation and/or position ofimplants in the eye will be described.

a) Location of the Implant Markings in the Eye (Toric Marks orMultifocal Marks)

Different eye implants like toric IOLs or Multifocal IOLs do havedistinct markers. According to one embodiment these markers areautomatically detected using image processing techniques, e.g. edgedetection and/or template based feature detection. This way basicallyany man made feature on or in an inlay or implant can be detected andtheir lateral position in the eye can be monitored over time.

In the case of toric IOLs e.g. the markings do show either the steep orthe flat axis of the toric lens and they are used by the surgeon toaccurately align the lens in the eye. In case of multifocal IOLs,concentric rings in the lens are visible which are used by the surgeonto laterally position the lens. FIG. 10 illustrates these markings andtheir determination in an eye image.

b) Cyclotorsion Orientation of Implants

As mentioned above the cyclotorsional orientation of a toric IOL can berecovered by detecting the toric marks on the lens that resemble eitherthe flat, the steep or implantation axis of the IOL (depending on thetype). This is also illustrated in FIG. 10 by the axis that is overlaidover the steep or the flat axis of the toric lens and which have beendetermined based on the location of these markings.

c) Roll and Tilt Orientation of Implants

The exact shape and refraction of the implant (for example an IOL) isknown. This allows for a model based ray tracing approach to recoverroll, tilt orientation and lateral position of the IOL in the eye, whichis used according to one embodiment to determine the roll and tilt of animplant.

The known coaxial illumination system of the RD creates reflections onthe front side and backside of the IOL (3rd and 4th order purkinjeimages), as illustrated in FIG. 11. If the lens rolls or tilts, the 3rdand 4th order purkinjes will move with respect to each other. In thespecial case in which the 3rd and 4th order purkinje superimpose, theoptical axis of the IOL is aligned with the optical axis of the camera.The locations of the 3rd and 4th order purkinje images can be used todetermine the roll and tilt of the implant, e.g. by using an approach asdescribed in “Reproducibility of intraocular lens decentration and tiltmeasurement using a clinical Purkinje meter”, Yutaro Nishi et. al. JCataract Refract Surg 2010; 36:1529-1535 Q 2010 ASCRS and ESCRS.Reference is in this context also made to FIG. 12 which illustrates thedetermination of the determination of the orientation of the intraocularlens based on the 3rd and 4th order purkinje reflections. Like in theusage of the reference device before a circular illumination is appliedwhich is coaxial with the camera axis. The orientation determinationmethod in one embodiment then may comprise the following steps:

1. Detect center of purkinje 3rd

2. Detect center of purkinje 4th

3. Use the IOL shape information including distance between anterior andposterior centers of curvature=DCC

4. Recover optical axis of lens using distance between 3rd and 4thpurkinje centers, camera parameters and DCC.

d) Implant Contour

The implant contour is clearly visible in the RD images if it is notobstructed by iris tissue. The unobstructed parts can be recovered withstandard image processing techniques like edge detection. By fitting aknown edge shape model of the inlay in the detected contour parts ordetected implant markings in one embodiment it is also possible torecover the obstructed parts of the inlay contour. This is illustratedin FIG. 13.

e) XY-Position of the Implant Center

Since the shape of the implant is known a variety of techniques can beused to recover the lateral position of the implant center. According toone embodiment, detecting the location of the implant marks, using theimplant contour to recover the center or the ray tracing approachdescribed under c) can be used.

f) Location of the Implant Haptics in the Eye

For detecting the haptics according to one embodiment the same approachas for the implant contour is employed. The haptics have a well definedshape and are basically part of the implant contour. Now someembodiments where the implant related parameter relates to the rhexiswill be described.

g) Contour

Using edge detection techniques can recover the clearly visible rhexisin the RD images. Alternatively it can also be measured by manuallyselecting a polygon that best resembles the contour of the rhexis. Therhexis contour is illustrated in FIG. 14.

h) Diameter

The diameter can be retrieved by least squares fitting of a circle orellipse into the contour of the rhexis.

i) XY Position in the Eye

The XY position of the rhexis according to one embodiment can be definedand determined as the center of the best fit circle or ellipse into thecontour of the rhexis.

j) Overlap with Lens

Superimposing the contour of the rhexis with the contour of the lens.The area inside the contour of the lens implant and outside the contourof the rhexis is the overlap. This is illustrated in FIG. 15. This is animportant measure to determine how stable the lens implant is in theeye. If the overlap on one side becomes too small chances are theimplant will be instable.

In the foregoing several embodiments of the invention have beendescribed which come along with several advantages.

E.g. by being able to spatially transform all measurements to an initialreference frame (or any arbitrary reference frame chosen in one of thesessions), any influence due to a potential eye motion can be eliminatedand all measured parameters can be normalized with respect to thereference frame.

This allows a continuous monitoring of all measured eye parameters. Atruly measurement driven approach to investigate the post surgerybehavior of implants and surgical cuts in the eye becomes possiblewithout being limited in accuracy to the amount of eye motion inherentlypresent in all multi session diagnostic data collection trials.

The skilled person will recognize that the modules or units of theembodiments of the invention described before may be implemented bysoftware or hardware or a combination thereof. In particular, thehardware may comprise a camera and a computer which is programmed toperform the tasks as described in connection with the embodiments of theinvention, in particular such tasks as image processing to determine eyeparameters or displaying for displaying eye parameters in addition tothe eye image.

The invention claimed is:
 1. A method for monitoring at least onesurgical parameter of an eye of a patient over multiple sessions whichare temporally spaced apart and between which the eye can have moved,the method comprising: determining, during a first session, the at leastone surgical parameter of the eye and its coordinates based on an imagetaken by a camera in a first coordinate system; determining, during asecond session temporally spaced apart from the first session, the atleast one surgical parameter of the eye and its coordinates based on theimage taken by a camera in a second coordinate system; determining aneye motion in six degrees of freedom between the first session and thesecond session and determining a coordinate transformation basedthereon; transforming, based on the determined eye motion, the at leastone surgical parameter of the eye and its coordinates from the firstcoordinate system into the second coordinate system; and quantifying orvisualizing a change of the at least one surgical parameter of the eyeand its coordinates between the first session and the second sessionbased on the at least one surgical parameter of the eye and itscoordinates measured during the second session and the transformed atleast one surgical parameter of the eye and its coordinates measuredduring the first session; wherein the at least one surgical parameter ofthe eye comprises one or more of the following: implant-relatedparameters of the eye which are based on an implant which has beensurgically placed in the eye of a patient; and a location and/or contourof corneal or limbal or scleral incisions.
 2. The method of claim 1,wherein the implant related eye parameter comprises one or more of thefollowing: an orientation or position of the implant in the eye; alocation or the contour of a rhexis; an overlap of the rhexis with acontour of the implant.
 3. The method of claim 1, further comprising:displaying the least one surgical parameter of the eye measured duringthe second session and the transformed at least one surgical parameterof the eye measured during said first session in the image of the eyetaken during the second session; and calculating a difference betweenthe at least one surgical parameter of the eye measured during thesecond session and the transformed at least one surgical parameter ofthe eye measured during the first session and visualizing the differencein the image of the eye taken during the second session.
 4. The methodof claim 1, wherein the first session is one of: a pre-surgery session,wherein the second session is an intra surgery session or a post surgerysession, an intra-surgery session, wherein the second session is a postsurgery session, and a post-surgery session, wherein the second sessionis another post surgery session performed at a later time.
 5. The methodof claim 1, further comprising measuring and recording the at least onesurgical parameter of the eye during multiple sessions over time inorder to record the change of the at least one surgical parameter of theeye over time.